Tobias Hanrath wants to harness the sun.
The Earth receives about 200 terawatts of usable energy from the sun every day, more than 10 times mankind’s daily consumption. But solar cells haven’t replaced fossil fuels because current photovoltaic technology is so inefficient and expensive to produce that the cost per kilowatt hour is too high.
The efficiency of the best photovoltaics today tops out at around 16 percent, but Hanrath thinks the unusual behavior of materials at infinitesimal scales can change all that. “We’re trying to address the key inefficiency of solar cells with nanoscale materials,” he says. “We hope to break the notorious 30 percent efficiency limit with these.”
Hanrath is working with crystalline silicon, the same semiconductor material found in conventional solar panels, but at such small scales that its fundamental behavior changes. In large silicon crystals, one incoming photon frees one electron. But because silicon nanocrystals are smaller than the wavelength of light, the photon becomes trapped, knocking several electrons loose as it bounces around. “At dimensions smaller than 10 nanometers, the quantum confinement effect becomes really important,” says Hanrath. “This process is known as multiexciton generation.”
Deposited in thin films, silicon nanocrystals are not only more efficient, they can also be used to create flexible solar cells. Making them, however, can be tricky. “Silicon nanocrystals are actually a bit of a challenge to make,” says Hanrath, “but it’s a chemical engineering challenge and we’re well equipped to address it.”
Making simple adjustments to nanocrystal size, shape, composition, and surface chemistry, Hanrath hopes to find the most efficient material for solar cells. “There are different knobs you can tune and we’re trying to figure out the correlation between changing properties and changing efficiency,” he says. “You can basically build materials with the properties you like.”
Hanrath is also investigating how nanocrystals might be used in batteries. Current technologies require separate devices for energy conversion and storage, which adds cost and weight. “The ultimate dream is to have both technologies in a single device,” he says. “You could have a thin-film solar cell and a thin-film battery.”
Hanrath has recruited four students, including two undergrads, to his group so far and they’ve already given him new energy. “I’ve been really amazed with their level of enthusiasm,” he says. “Seeing that excitement in them has been very invigorating for me.”
Hanrath was told how easy it is at Cornell to work with researchers from different departments and he is looking forward to developing new collaborations. “Of course, people only tell you the best things when you interview, but it turns out here it’s actually true,” he says. “It’s a very exciting time to be involved in energy related materials research.”
Prof. Hanrath's Web page
